Since the invention of electric light, the night has been getting progressively brighter. But a growing body of research shows experiencing total darkness is critical to our well-being. Throughout history, humans have gathered in the dark to marvel at the starry blanket overhead. Nowadays, artificial light pollution obscures the night sky in much of the industrial world—which could lead to unforeseen consequences for the environment and for human health.

Since the invention of electric light in the 19th century, the night has been getting progressively brighter, but things really took off in the 1990s. The acceleration has to do with population growth, an increase in nighttime labor, and the widespread belief that more light makes cities safer. Light does increase safety, but only up to a point. Too much light at night can actually make cities more dangerous. With overlit gas stations and bright LED streetlights shining skyward, a clear night sky is harder and harder to find. And our eyes, which developed the ability to see in low-light settings, rarely have the occasion to use that skill anymore.

Forty percent of Americans and Western Europeans never or rarely experience night vision. We’re in the light so much that our eyes never switch. Even inside it’s hard to avoid the glow of streetlights, and we often glance at brightly lit cellphone screens right up until we fall asleep.

There’s a growing medical consensus that all this artificial light is bad for our health. It interrupts our sleep patterns, confuses our circadian rhythms, and inhibits our ability to produce melatonin. Melatonin is only produced in the dark, and what scientists are finding is that a lack of melatonin in our bloodstream is linked to an increased risk for breast and prostate cancer.

Doctors are not saying that the light in your cellphone will give you cancer, but increasingly they recognize the importance of darkness to our overall wellbeing. We’ve evolved in bright days and dark nights, just like all life on Earth, and we need both for optimal health.

Wildlife also depends on darkness. Sea turtles, for example, need a dark sky to navigate. When hatchlings climb out of their nest on the beach, they need to crawl their way to the sea. They’ve evolved to swim or scurry to the brightest light on the horizon, which for all those hundreds of millions of years has been the stars and the moon reflected on the water, but now it’s the hotels and parking lots in the wrong direction. Concerned about the impacts of light pollution, a growing movement is working to reduce excess lighting in our cities and protect dark skies.